How would I Mask the entire Stage?

Hello.
I searched around and could not find the answer I am looking for.
I have a fullscreen flash project.
Basically what I need to know how to do is resize the stage height and mask everything to the new height (I have a full page scrollbar, and it scroll to over 2x the height I want. And the height will change with each section)

Is there a way to do this? There must be!
Thanks.